chore: configure Dokploy Railpack deployment

Railpack's Node provider builds this Astro project as a static site with zero
config (bun install -> bun run build -> Caddy serving dist), verified with
`railpack plan`:

- railpack.json pins provider=node explicitly, since the repo has both a
  package.json and a public/ dir, and sets TZ=UTC
- Staticfile sets root=dist and index_fallback=false. Railpack's default SPA
  fallback appends /index.html to try_files, which answers unknown URLs with the
  homepage and HTTP 200; disabling it yields a real 404 for this multi-page site
- src/pages/404.astro gives the static build a dist/404.html to serve
- build script now runs `astro check && astro build`, so a type error fails the
  deploy instead of shipping

Dokploy side: build type Railpack, container port 80 (the generated Caddyfile
listens on {$PORT:80}), and GET /health returns 200 for healthchecks.

Staticfile

@@ -0,0 +1,12 @@
+# Read by Railpack (Dokploy "Railpack" build type) to configure the Caddy server
+# that serves the Astro static build produced by `bun run build`.
+# Docs: https://railpack.com/languages/staticfile/
+
+# Directory Caddy serves (Astro's build output).
+root: dist
+
+# This is a multi-page site: /, /add and /edit are real files on disk.
+# index_fallback: true (Railpack's default) would answer unknown URLs with
+# index.html and a 200 (soft 404). Disabling it makes unmatched paths return a
+# real 404, served from Astro's generated dist/404.html.
+index_fallback: false